The present invention in its most general aspect relates to filtration methods and apparatus for same. More particularly, it relates to filtration methods employing semi-permeable membranes as the filtration media. Such methods include reverse osmosis, ultrafiltration, dialysis, electro-dialysis, water-spitting, pervaporation and microfiltration, and depend on certain components being much more permeable through the membrane than other components. The result of such filtration methods is to separate one or more substances by retaining some within a retentate while others are separated into a permeate. In the case of electrodialysis, certain ions are much more permeable through the membrane than other solution components. The valuable fraction may be either the concentrate, the permeate or, in some instances, both, depending on the particular application. In the microfiltration of beer and the desalination of brines, the permeate is the desired product. In the preparation of pharmaceutical intermediates wherein bacterial growth is carried out in an inert but acceptable medium, the concentrate is the valuable fraction.
In its most specific aspect, the present invention is directed at overcoming the ubiquitous problem of clogging or blinding of the filter media by the accumulation of dissolved or suspended material thereon. As pointed out in detail below, in accordance with the present invention this is accomplished by a unique application of a known hydrodynamic phenomenon or principle, which is accompanied by several surprising and unexpected benefits, also set forth below.
Understanding of the invention will be facilitated by an initial consideration of filtration processes in their most general aspects. In terms of worldwide annual tonnages, mineral benefication processes are among the most prevalent of methods utilizing filtration. Typically, a mineral-bearing ore is ground to desired fineness and mixed with water and a variety of surface-active chemicals. Then, in a flotation cell, air is bubbled through the mixture, and the chemicals act to attach the non-mineral-bearing lighter particles to the bubbles, which form a froth on the surface. The concentrate and liquid, after removal of the froth, are passed over large rotating drums having coarse screens covered with a special canvas, and water is drawn out through the center of the drum with suction, and a filter cake is separated from the drum exterior, on each revolution and continuously, with a doctor blade. Fine particles do find their way into the canvas, however, and this clogs or blinds the filter, preventing the separation from occurring and requiring a back-wash operation to continue the process (see, generally, Fuerstenau, Ed., "Flotation," Am. Inst. of Min. Met. & Pet. Engs., New York, 1976).
Interestingly, the same principles that govern mineral separation in huge froth flotation plants handling millions of tons of ore also apply to separations carried out in laboratories on centiliters of a raw solution using the most sophisticated equipment. Further, the same principles apply to separations of true solutions with no particulates involved (e.g., solutions with dissolved molecules such as salts, proteins, etc.). Of course, the filter media and the process conditions are more different, but the basic principles are the same.
In conventional separations with a stationary membrane the first condition is that the liquid mixture pass over a large area of filter media in a short time. The reason for this is apparent; total flux through the membrane is proportional to its area, and separation will occur only at the liquid-media boundary, ofter called the boundary layer. This boundary layer tends to retain rejected solute species, which are retarded from returning to the bulk solution. This leads to concentration polarization and in some cases to formation of gel layers.
Of course, what has made more fine separations possible is the development of sophisticated filter materials, known generally as semi-permeable membranes. In microfiltration, for example, micro-sized pore filters can filter out the bacteria that would otherwise spoil unrefrigerated beer, replacing pasteurization and making available storage-stable "real draught" beer in markets. In reverse osmosis filtration, brines and other polluted solutions can be rendered potable (usually after several treatments in seriatim) provided the system pressure on the filter medium exceeds the osmotic pressure. Specifically, tailored plastic and cellulosic materials form the filter media in such cases. In electro-dialysis filtration, similar media are used, but an electric charge--creating an effective cathode and anode--help propel the separation. Also, therapeutic dialysis is used to purify patient's blood. However, present systems are very expensive and are of limited availability.
As noted above, the clogging or blinding of filter media is a problem at any level of filtration, insofar as transmembrane flow (flux) drops as the pores in the filter media become clogged. While scraping off a filter cake and backwashing the canvas will suffice in flotation separations, the problems multiply when one deals with finer separations. Gels (highly hydrated molecules also called flocs) can form. As solute concentration builds up at the boundary layer, chemical precipitation of colloidal-size particles can occur, a typical case being the precipitation of gypsum (hydrated calcium sulfate) from sea water. Blinding and clogging problems are compounded by the fact that semi-permeable membranes are not amenable to the rough treatment accorded drum filters in ore-dressing plants. How prior workers have addressed this problem is set forth below.
Huntington, in U.S. Pat. No. 3,355,382, discloses a reverse osmosis desalination system that rotates at over 1,000 r.p.m., providing G forces of 400, 300 and 200 on three concentric membranes. The outer shell and membranes rotate as a unit. The device does not fly apart from centrifugal rupture because of hydraulic pressure on the membranes, around 1,000-2,000 p.s.i.a. The membranes are periodically cleaned by suddenly closing a valve in the outlet line to momentarily raise the back pressure above the brine pressure, creating a "water hammer." In U.S. Pat. No. 3,396,103 of the same inventor, the cylindrical membranes are replaced by planar segments, and the resulting variation of centripetal acceleration across the segment surfaces is said to force transverse lateral circulation of brine, and lower boundary layer solute concentration.
The reverse approach is taken by Grenci in U.S. Pat. No. 3,400,074. Here, the brine is fed to the interior of the drum and the acceleration achieved by rotation is used to drive the fluid against the peripheral membrane and thus overcome the osmotic pressure. The membrane and drum rotate as a unit. This patent does not really address either the problem of membrane blinding or the structural integrity of the membrane, though in the latter instance it mentions that inlet and outlet pressures are low, but that pressure on the membrane can range from 10 to 10,000 p.s.i.
U.S. patents to Manjikian, U.S. Pat. Nos. 3,821,108, 3,830,372 and 3,849,305 disclose long, narrow-diameter cylindrical membrane units eccentrically mounted in a fixed cylinder along with mechanical stirrers, rotation of the stirrers being relied on to maintain a turbulent condition that will keep the membranes free of occluded matter. The cylindrical membrane units are displaced in a circular path about the axis of the fixed cylinder.
The British patents of Keefer, Nos. 1,603,746 and 1,603,747, disclose a first rotor which imparts an angular velocity to the feed and a second rotor, including a diffuser and the membrane, which operates at a distinct, lower speed. By encouraging free convection of the feed, concentration polarization and filter clogging is reduced.